Supercapacitors are being considered for electric vehicle (EV), renewable energy storage, and modern grid applications. The high volumetric capacitance density of a supercapacitor (10 to 100 times greater than those of electrolytic capacitors) derives from using porous electrodes to create a large surface area conducive to the formation of diffuse double layer charges. This electric double layer (EDL) is created naturally at the solid-electrolyte interface when voltage is imposed. This implies that the specific capacitance of a supercapacitor is directly proportional to the specific surface area of the electrode material, e.g. activated carbon. This surface area must be accessible by electrolyte and the resulting interfacial zones must be sufficiently large to accommodate the EDL charges. In some supercapacitors, the stored energy is further augmented by pseudo-capacitance effects due to some electrochemical reactions (e.g., redox).
Since the formation of EDLs does not involve a chemical reaction, the charge-discharge process of an EDL supercapacitor can be very fast, typically in seconds, resulting in a very high power density (typically 5,000-10,000 W/Kg). Compared with batteries, supercapacitors offer a higher power density, require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, experience no “memory effect,” and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life.
Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for various industrial applications. For instance, supercapacitors possess very low energy densities when compared to batteries (e.g., 5-8 Wh/kg for commercial supercapacitors vs. 10-30 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). On the other hand, lithium-ion batteries possess a much higher energy density (100-180 Wh/kg), but deliver a very low power density (100-500 W/Kg), requiring typically hours for re-charge. Conventional lithium-ion batteries also pose some safety concern.
The low power density or long re-charge time of a lithium ion battery is due to the mechanism of shuttling lithium ions between a negative electrode (anode) and a positive electrode (cathode), which requires lithium ions to enter or intercalate into, for instance, inter-planar spaces of a graphite crystal at the anode during re-charge, and into the complex cathode crystals (e.g. lithium cobalt oxide, lithium iron phosphate, or other lithium insertion compound) during discharge. This intercalation or diffusion process requires a long time to accomplish because solid-state diffusion (or diffusion inside a solid) is difficult and slow. For instance, the current lithium-ion battery for plug-in hybrid vehicles requires 2-6 hours of recharge time, as opposed to just seconds for supercapacitors. The above discussion suggests that an energy storage device that is capable of storing as much energy as in a battery and yet can be fully recharged in one or two minutes like a supercapacitor would be considered a revolutionary advancement in energy technology.
Instead of using an inorganic lithium insertion compound, such as LiCoO2 and LiFePO4, that requires lithium insertion into and extraction from a bulk inorganic particle (typically 100 nm-20 μm, but more typically 1-10 μm), several attempts have been made to use organic molecules or polymers as an electrode active material for the cathode (lithium metal as the anode). For instance, Le Gall, et al investigated Poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene) as an organic polymer cathode [T. Le Gall, et al. J. Power Sources, 119 (2003) 316-320] and Chen, et al used LixC6O6 organic electrode, obtained from a renewable source, in a lithium ion battery [H. Chen, et al. “From biomass to a renewable LixC6O6 organic electrode for sustainable Li-ion batteries,” ChemSusChem, 1 (2008) 348-355]. Unfortunately, these organic materials exhibit very poor electronic conductivity and, hence, electrons could not be quickly collected or could not be collected at all. Although these organic molecules contain carbonyl groups (>C═O) that presumably could readily react with lithium ions (forming a redox pair), this redox mechanism was overwhelmed by the poor electronic conductivity. As a result, the battery cells featuring these organic molecules exhibit poor power densities. Le Gall et al added a large proportion of conductive acetylene black (typically 40-60% by weight) to partially overcome the conductivity issue; but, acetylene black significantly dilutes the amount of the active material. Further, the best achievable specific capacity of 150 mAh/g is far less than the theoretical specific capacity of 705 mAh/g of Poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene).
Recently, more electrically conducting carbon nano-tubes (CNTs) containing carbonyl groups were used by Lee, et al to replace the organic molecules for use as a cathode material [S. W. Lee, et al, “High Power Lithium Batteries from Functionalized Carbon Nanotubes,” Nature Nanotechnology, 5 (2010) 531-537]. The significantly higher electronic conductivity of CNTs does serve to overcome the poor conductivity problem of organic molecules. However, the CNT-based approach still suffers from several relatively insurmountable technical and economical issues, which call into question the commercial viability or utility value of this approach. Some of these issues are:                (1) CNTs are known to be extremely expensive due to the low yield, low production rate, and low purification rate commonly associated with the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs.        (2) CNTs tend to form a tangled mess resembling a hairball, which is difficult to work with (e.g., difficult to disperse in a liquid solvent or resin matrix).        (3) The so-called “layer-by-layer” approach (LBL) used by Lee, et al is a slow and expensive process that is not amenable to large-scale fabrication of battery electrodes, or mass production of electrodes with an adequate thickness (100-300 μm thick).        (4) The CNT electrodes prepared by the LBL process have their thicknesses in the range of 0.3-3.0 μm. Unfortunately, the data provided by Lee, et al (e.g. FIG. S-7 of the Supporting Material of Lee, et al) show that the power density dropped by one order of magnitude when the LBL CNT electrode thickness was increased from 0.3 μm to 3.0 μm. A useful battery or supercapacitor electrode thickness is typically in the range of 50-500 μm (more typically 100-300 μm).        (5) Although the ultra-thin LBL CNT electrodes provide a high power density (since Li ions only have to travel an extremely short distance), there was no data to prove that CNT-based electrodes of practical thickness could even work due to the poor dispersion and electrolyte inaccesability issues. Lee, et al showed that the CNT-based composite electrodes prepared without using the LBL approach did not exhibit particularly good performance.        (6) CNTs have very limited amount of suitable sites to accept a functional group without damaging the basal plane or graphene plane structure. A CNT has only one end that is readily functionalizable and this end is an extremely small proportion of the total CNT surface. By chemically functionalizing the exterior basal plane, one would dramatically compromise the electronic conductivity of a CNT.        
Hence, there exists an urgent need to develop a new class of highly conducting electrode materials having a functional group that is capable of rapidly and reversibly forming a redox reaction with lithium ions. These materials must have an adequate amount of readily functionalizable sites to host a desired amount of useful functional groups. These materials must be mass-producible with low costs. These materials must be stable in a wide temperature range (e.g. −40° C. to 60° C., a commonly operating range for a battery). After an extensive and intensive research and development work, we have discovered a new type of electrode materials that meet this set of stringent technical and economical requirements.
This new type of materials includes a chemically functionalized disordered carbon material having certain specific functional groups capable of reversibly and rapidly forming a redox pair with a lithium ion during the charge and discharge cycles of a battery cell. The functionalized disordered carbon is used in the cathode (not the anode) of the presently invented lithium super-battery. In this cathode, lithium ions in the liquid electrolyte only have to migrate to the edges or surfaces of aromatic ring structures or small graphene sheets in a disordered carbon matrix. No solid-state diffusion is required. In contrast, carbon or graphite is used as an anode active material in current or prior art lithium-ion batteries, wherein lithium ions must diffuse into and out of the inter-graphene spaces of a graphite crystal.
Specifically, two types of disordered carbon (soft carbon and hard carbon) have been commonly used in the anode of conventional lithium-ion batteries, wherein inter-planar spaces in a graphite crystal serve as a host for lithium ions to intercalate in and out. This energy storage mechanism in the anode is based on lithium intercalation into the crystal structure. Further, in the conventional lithium ion battery, the cathode active material is typically a lithium intercalation compound, such as LiCoO2 and LiFePO4, rather than a carbon or graphite material. In the conventional lithium ion battery, lithium ions or atoms are intercalated or inserted into the interior crystal structure of a carbon/graphite anode (during the charging procedure) or a non-carbon-based lithium intercalation compound (during discharge). This insertion or extraction procedure is slow. Due to this slow process of lithium diffusion in and out of these intercalation compounds (a solid-state diffusion process), the conventional lithium ion batteries do not exhibit a high power density and the batteries require a long re-charge time. None of these conventional devices rely on select functional groups (e.g. attached at the edge or basal plane surfaces of a graphite crystal in a non-crystalline carbon matrix) that readily and reversibly form a redox reaction with a lithium ion from a lithium-containing electrolyte.
In contrast, the presently invented lithium super-battery relies on the operation of a fast and reversible reaction between a functional group (attached or bonded to a disordered carbon structure) and a lithium ion in the electrolyte. Lithium ions coming from the anode side through a separator only have to diffuse in the liquid electrolyte residing in the cathode to reach a surface/edge of a cathode active material domain. These lithium ions do not need to diffuse into or out of a solid particle. Since no diffusion-limited intercalation is involved, this process is fast and can occur in seconds or even shorter. Hence, this is a totally new class of hybrid supercapacitor-battery that exhibits an unparalleled and unprecedented combined performance of an exceptional power density, high energy density, long and stable cycle life, and wide operating temperature range. This device has the best of both worlds (battery and supercapacitor).